1. Field of the Invention
The invention relates to a slurry of sulfur in a liquid carrier comprising at least one biodiesel, the use of the sulfur as a cross-linking agent for cross-linkable molecules, and asphalt compositions containing asphalt and sulfur/biodiesel slurry.
2. Description of the Prior Art
Road building has been a source of inspiration and aggravation for civilization for millennia. Ancient Rome built roads that would last for centuries, but which required an enormous investment in labor and materials and a high level of sophistication. In the 1830""s, travelers prized the opportunity to travel on short stretches of macadam roads made of three layers of graded stone, the top layer of which contained some bituminous or asphaltic binder.
The need to reduce costs and use locally available materials led to wooden plank roads, which were, in turn, replaced with asphalt roads. By the early 20th century, asphalt roads, mixtures of gravel and asphaltic petroleum fractions, were the dominant roads. Many times an asphalt gravel mix/was simply poured on a minimally prepared surface. Such roads could be built cheaply and quickly with unsophisticated labor, but such roads deteriorated rapidly.
Roman roads lasted for centuries, but took decades to build. Macadam roads, built according to the original rigid specifications of 1830, lasted for decades, but took months or years to build. Many asphalt roads were built in weeks or months only to deteriorate after a few years. Asphalt roads that deteriorate rapidly do so because of lack of surface preparation and drainage, poor construction practices, and poor quality materials.
Asphalt roads have come full circle. The early macadam roads were prized for their quality and durability. Many modern macadam roads suffer a largely undeserved poor reputation for quality.
There is a renewed interest in building quality roads with asphalt as civil engineers and municipalities realize that better asphalt roads are possible by using better surface preparation, construction practices, and better asphalts.
Some improvements in asphalt properties were achieved by selecting the starting crude petroleum, or control of the refinery processing steps used to make the asphalt. Unfortunately, there are many crudes which do not make good asphalts. There are only a limited number of steps which can be taken to control the refining process to make better asphalt.
The next step taken by the industry was to modify the asphalt. Air blowing makes asphalts harder. Fluxing agents or diluent oils are sometimes used to soften the asphalt.
Marked changes in asphalt properties can be achieved with sulfur, either added to neat asphalt or when added as a cross-linking agent to treat a mixture of asphalt and polymer.
The conventional methods of adding sulfur (dumping loose powder or bags of powdered sulfur on top of molten asphalt in a mix tank) had some problems. Of significant concern is safety. There exists the potential for fire and explosion hazards, caused by having a potentially large cloud of hot and flammable sulfur dust. Sulfur dust is by its very nature considered explosive.
It is also difficult in a commercial facility to rapidly and completely mix powdered sulfur with asphalt. Some parts of the asphalt contained too much sulfur too rapidly while other parts of the blend were sulfur deficient. Such an approach, led to over cross-linking parts of the blend, forming lumps of super-vulcanized materials which were not compatible with the rest of the blend.
U.S. Pat. No. 6,153,004, Hayner and U.S. Pat. No. 6,133,351, Hayner, which are expressly incorporated herein by reference, disclosed use of a sulfur slurry for sulfur addition to asphalt, rather than dry, powdered sulfur. In a preferred embodiment, the finely ground sulfur was mixed with a portion of a flux oil which would be added to the asphalt. The ""004 and 351 patents described the use of suitable flux oils found in myriad refinery streams, including various neutral oils, aromatic extract, vacuum gas oil and refinery bottoms streams.
While the sulfur slurry method taught in the ""004 and ""351 patents was a significant improvement over the state of the art in asphalt manufacture, there are some areas where further improvement is still needed. Sulfur slurries, if not used fairly promptly after manufacture, can settle into a S rich sludge and a S lean oil phase above the sludge. Remixing (or prompt use) solves this problem, but it is a nuisance and time-consuming, which, in turn, raises the costs.
One attempt to solve these problems included the use of relatively heavy liquid refinery streams, which reduced hydrocarbon vapor pressure and made the sulfur slurry safer to use, but produced relatively viscous slurries which were hard to pump.
There are also concerns about, and some localities have limits on, exposure of workers to aromatics. Essentially all refinery streams heavier than gasoline have significant amounts of aromatics present, unless the aromatics have been removed or hydrotreated so severely as to saturate the aromatics.
In addition, the conventional refinery liquid streams can have a significant vapor pressure, leading to risk of fire and even, in some cases, of explosion. Although the risk of explosion can be eliminated, and the risk of fire greatly reduced by using a higher boiling liquid hydrocarbon stream, the higher boiling streams are more viscous and difficult to pump.
The present invention provide a slurry of sulfur in a liquid carrier comprising as ester derived from vegetable oils or animal fats. In one aspect, the present invention relates to the use a natural biodiesel, derived from vegetable oils or animal fats, as the liquid part of the sulfur slurry. So far as is known, biodiesel has never been used to form a sulfur slurry until the present invention. Rather, the only use of biodiesel to form a slurry was described in Development of Biodiesel Slurry Fuels, reported on page 57 of Biodiesel Research Progress 1992-1997, K. Shaine Tyseon, Editor, available on WWW at http://www.ott.doe.gov/biofuels/pdfs/biodiesel 92-97.pdf, which paper is incorporated herein by reference. The slurry fuel described therein was finely ground (LT 100 micron) corn or sorghum in biodiesel.
Biodiesel is now well known and widely available commercially and extensively studied, as shown by over 100 papers included in the report, Biodiesel Research Progress, discussed above. Biodiesels are attractive for fuels, and some other uses, because they have a low vapor pressure, are non-toxic and are stable, as per HMIS regulation, and do not deteriorate or detonate upon mild heating. Chemically, biodiesels are generally defined as the mono alkyl esters of long chain fatty acids derived from renewable lipid sources.
Although these benefits (safety, toxicity) are characteristic of biodiesel, it was not until the present invention that the inventors herein thought to look at biodiesel as a suitable vehicle for getting sulfur into an asphalt blend. In particular, there were some concerns about compatibility of biodiesel in asphalt. Asphalt is high molecular weight and highly aromatic. In contrast, biodiesel is aliphatic, contains no sulfur, has no ring structures or aromatics, and is relatively in low molecular weight, as compared to asphalt. Also, biodiesel also contains large amounts of oxygen, approaching 10%. Until the present invention is was believed that a linear, relatively low molecular weight, aliphatic molecule such as biodiesel would be better for precipitating asphalt from crude oil than for efficiently dispersing a sulfur slurry into asphalt.
Despite these concerns, it was surprisingly found by the inventors herein that the aliphatic biodiesel sulfur slurry was completely satisfactory. The use of biodiesel provides the benefits of fire hazard safety and low toxicity and is a satisfactory vehicle for the sulfur particles. In addition, the sulfur-in-biodiesel suspension is relatively stable and does not separate as rapidly as do sulfur slurries that contain petroleum refinery derived liquid streams. The asphalt did not xe2x80x9crejectxe2x80x9d the biodiesel sulfur slurry; that is, there were no problems with phase compatibility, and the biodiesel sulfur slurry could rapidly be mixed with the asphalt. The biodiesel sulfur slurry of the present invention has a relatively low viscosity, at least lower than the refinery liquid streams previously being used, and is easy to pump.
While we do not wish to be bound by any theory of why use of biodiesel sulfur slurry gives superior results, we believe the biodiesel additive facilitates dispersion of sulfur within the mixture. The biodiesel is a fatty acid reacted with an alcohol, and the reaction product (the ester) tends to xe2x80x9cwetxe2x80x9d both sulfur and aggregate better than typical fuel oils. Wetting agent is probably not necessary to disperse the sulfur in liquid to form the slurry.
The asphalt produced from the biodiesel sulfur slurry also seems to coat the stone better than similar asphalt made with a sulfur slurry comprising a fuel oil as the liquid.
Most surprisingly, the use of biodiesel sulfur slurry leads to an apparent increase in polymer concentration when a biodiesel sulfur slurry is used as a cross-linker for polymer/asphalt blends. The biodiesel sulfur slurry of the present invention is readily dispersible in the asphalt and does not lead to over- or under- cross-linked parts of the asphalt blend, would have been expected before this invention. It was not expected that the biodiesel sulfur slurry would make the polymer blended in the asphalt more effective, but this is what the data shows.
Also, it should be understood that the polymer is the most expensive part of the asphalt blend, and by use of the present invention, it is now possible to reduce the amount of this expensive component, or, alternatively, to increase its effectiveness, by changing the solvent for the sulfur slurry to comprise biodiesel.
The present invention also provides an improved manufacturing process by reducing the amount of sulfur settlement that occurs in an asphalt mixture, which, in turn allows for easy remixing of the asphalt mixture. Also, the improved distribution of sulfur in the polymerized asphalt cement manufacturing reduces the amount of raw polymer required to meet finished product specifications. The present invention also provides a less viscous sulfur slurry mixture which is easier to pump.
In another aspect, the present invention provides a method of preparing a mixture of asphalt, polymer and sulfur comprising blending together at asphalt blending conditions said asphalt and a slurry of sulfur particles in a liquid carrier comprising an ester derived from vegetable oils or animal fats.
In yet another embodiment, the present invention provides a method of enhancing the effectiveness of particles of sulfur as a cross-linking agent for a cross-linkable molecule, preferably at least one of aromatic molecules or polymer, comprising providing said sulfur as a slurry or sulfur in a liquid carrier comprising an ester derived from vegetable oils or animal fats.
In another aspect, the invention relates to a pavement composition comprising aggregate and from about 1.0% to about 10.0% of an asphalt composition comprising sulfur and biodiesel.